This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-103370, filed on Apr. 5, 2002; the entire contents of which are incorporated herein by reference.
The present invention relates to a heterojunction bipolar transistor and its manufacturing method, and more particularly, it relates to a heterojunction bipolar transistor being capable of a high-speed operation and having a high breakdown voltage and a high current gain and a method of manufacturing it.
In recent years, revolutionary development has accomplished in information technology including the Internet. For this reason, further improvement in the speed and capacity of wireless communications, such as an optical fiber communications system which makes that basis, and a cellular phone which makes that circumference, are required.
A transistor which is capable of high-speed operation is a key device of these high-speed communications. These transistors are made using semiconductors, such as Si (silicon) and GaAs (gallium arsenide). In a field for which more high-speed operation is needed, SiGe (silicon germanium), InP (indium phosphorus), etc. are capturing the spotlight as a next-generation material.
Further, high-power output is also needed for a device used for these optical communications and wireless communications besides high-speed operation. For this reason, improvement in resisting voltage of a transistor is also required. xe2x80x9cA double heterojunction bipolar transistor (DHBT)xe2x80x9d in which an emitter and a collector has a wider bandgap compared with a base is a hopeful transistor which fills this demand.
In the conventional general (single) heterojunction bipolar transistor (SHBT), base-collectors junction is a homojunction. That is, material with a comparatively small bandgap is used for the collector like the base. For this reason, in order to attain a high-breakdown voltage, it is necessary to make a collector thick enough. On the other hand, in DHBT, material with a wide bandgap is used not only for the emitter but for the collector. For this reason, thickness of the collector can be made thinner than SHBT. As the result, since electron transit time can be shortened, operation becomes possible at higher speed and a higher breakdown voltage.
By the way, it is known that there are two kinds of junction forms called xe2x80x9cType Ixe2x80x9d and xe2x80x9cType IIxe2x80x9d in a heterojunction. The xe2x80x9cType Ixe2x80x9d and xe2x80x9cType IIxe2x80x9d defined in this specification will be explained hereafter.
FIG. 12 is a band diagram showing a heterojunction of Type I. That is, this figure schematically shows a band structure of a heterojunction in an equilibrium state of two kinds of different semiconductors 12 and 14. Here, Ec shows a lower end of a conduction band, Ev shows energy of an upper end of a valence band, and Evac shows energy of a vacuum level as a standard of energy. Ec of the semiconductor 12 is located more closely to Evac than Ec of the semiconductor 14. Further, Ev of the semiconductor 12 is located remoter from Evac than Ev of the semiconductor 14. Such a heterojunction shall be called a heterojunction of xe2x80x9cType Ixe2x80x9d in this specification.
On the other hand, FIG. 13 is a band diagram showing a heterojunction of Type II. In this case, Ec of the semiconductor 22 is located remoter from Evac than Ec of the semiconductor 24.
Further, Ev of a semiconductor 22 is also located remoter from Evac than Ev of the semiconductor 24. Such a heterojunction shall be called a heterojunction of xe2x80x9cType IIxe2x80x9d in this specification.
FIG. 14 is a band diagram of a principal part of DHBT that has been embodied for a trial by the Inventors of the present invention in the course of attempting to make the invention complete. That is, this diagram shows the state where Emitter E, Base B, and Collector C are junctioned.
As shown to FIG. 14, in this DHBT, the base-collector heterojunction is of Type I shown in FIG. 12. As materials of the base and the collector, GaAs/InGaP, InGaAs/InP, etc. are used, for example. Thus, in the case of the DHBT which has the heterojunction of Type I in the base-collector interface, as typically shown in FIG. 14, potential barrier xcex94Ec exists between the collector and the base. Running of electrons is barred by this potential barrier and collector injection efficiency falls in a high current condition.
FIG. 15 is the so-called xe2x80x9cgummel plotxe2x80x9d showing the dependability of a collector current to the voltage VBE between the base and the emitter of the transistor. The curve B in this figure shows the characteristic of DHBT which has the band structure shown in FIG. 14. On the other hand, curve A shows the characteristic of a single-heterojunction bipolar transistor in which the same material is used for a collector and a base.
Since barrier xcex94Ec exists in the base-collector interface, it turns out that the collector current saturates at lower VBE in the curve B, than curve A. Thus, if the structure shown in FIG. 14 is used to obtain a high breakdown-voltage element, it is difficult to obtain a practically sufficient collector current. The outstanding performances, such as high-speed operation, high-level current gain, and high-level linearity can not fully able to be obtained.
As a means for avoiding this problem, it may be considered to connect the conduction bands of base and collector smoothly by making of the composition near the base-collector interface change gradually. Or by providing an intermediate layer containing high-concentration n-type impurities between base and collector, potential barrier width thereof may be decreased, and an electronic running may become easier by a tunneling effect.
However, in order to introduce such a composition inclination or an intermediate layer, it is necessary to perform a precise gas flow control when the semiconductor layer is grown by a Metal Organic Chemical Vapor Deposition (MOCVD) etc.
Consequently, a big burden is placed in equipment and its operation, and it is inferior in respect of manufacturing efficiency.
Further, even if the intermediate layer for obtaining the tunneling effect is introduced, it is difficult to obtain a sufficient current density.
On the other hand, it is considered to use Type II which is shown in FIG. 13 instead of Type I.
FIG. 16 is a band diagram of DHBT formed by using a heterojunction of Type II. By using the heterojunction of Type II, as shown in FIG. 16, the potential barrier between the base B and the collector C is lost. For this reason, unlike the case where heterojunction of Type I is used, decline in collector injection efficiency is eliminated, and DHBT having high collector injection efficiency can be realized.
However, as a result of a detailed examination of the Inventors, it turned out that in DHBT which had the type II heterojunction for emitter-base junction and base-collector junctions, there were some problems explained below.
First, if a junction of Type II is used, while a band barrier between base and collector will be lost, conversely, a band barrier arises between emitter and base, and a saturation value of collector current falls. As the result, since current density per unit area of the transistor decreases, it becomes necessary to enlarge size of the transistor to some extent in order to secure an output level.
When junction of Type II is provided between emitter E and base B, there maybe a problem that mobility of electrons in the base region falls. That is, as a trend of high-speed element development in recent years, in order to obtain low base resistance, impurities are needed to be added in the material of Base B in a high concentration.
When forming DHBT, GaAsSb and InGaAs can be mentioned as a typical material system in which high-concentration impurity doping is possible. However, in these material systems, the alloy scattering effect is severe, and since electrons injected into the base from the emitter experience remarkable scattering, mobility becomes extremely low.
For example, mobility of an electron in a material system mentioned above may fall to ⅕ of GaAs or less. As a result, a sufficient collector current may not be obtained at a predetermined voltage, and an operation speed may fall. That is, since electrons are injected into the base in the state near an equilibrium state when junction of Type II is provided between emitter E and base B, the problems may arise in response to such a scattering process.
Further, problems that collector injection efficiency may fall remarkably and current gain may become very low may arise, since the Auger recombination effect arising from high-concentration impurities is severe in these material systems.
Further, when using GaAsSb, selectivity over HCl which is the general etchant of InP is inferior to InGaAs. For this reason, there is also a problem that a reproducible process cannot be performed easily.
As explained above, it was difficult to obtain the transistor which has high speed, high breakdown, and high current gain.
According to an embodiment of the invention, there is provided a heterojunction bipolar transistor comprising, an emitter made of a first compound semiconductor of a first conductivity type; a base made of a second compound semiconductor of a second conductivity type and having a bandgap smaller than the first compound semiconductor; and a collector made of a third compound semiconductor of a first conductivity type and having a bandgap wider than the second compound semiconductor, the emitter and the base forming a heterojunction of type I, the base and the collector forming a heterojunction of type II, and the base including impurities by a concentration equal to or more than 5xc3x971019 cmxe2x88x923.
According to other embodiment of the invention, there is provided a heterojunction bipolar transistor comprising, an emitter made of a first compound semiconductor of a first conductivity type; a base made of a second compound semiconductor of a second conductivity type and having a bandgap smaller than the first compound semiconductor; and a collector made of a third compound semiconductor of a first conductivity type and having a bandgap wider than the second compound semiconductor, each of the first and third compound semiconductors includes a plurality of kinds of group III elements, and an orderliness of the plurality of kinds of group III elements in the third compound semiconductor is higher than an orderliness of the plurality of kinds of group III elements in the first compound semiconductor.
According to other embodiment of the invention, there is provided a heterojunction bipolar transistor comprising, an emitter made of a first compound semiconductor of a first conductivity type; a first base made of a second compound semiconductor of a second conductivity type and having a bandgap smaller than the first compound semiconductor; a second base made of a third compound semiconductor of a second conductivity type and having a bandgap smaller than the first compound semiconductor; and a collector made of a fourth compound semiconductor of a first conductivity type and having a bandgap wider than the third compound semiconductor, the third compound semiconductor having a composition that differs from a composition of the second compound semiconductor, the emitter and the first base forming a heterojunction of type I, and
the second base and the collector forming a heterojunction of type II.
According to other embodiment of the invention, there is provided a manufacturing method of heterojunction bipolar transistor having: an emitter made of a first compound semiconductor of a first conductivity type; a base made of a second compound semiconductor of a second conductivity type and having a bandgap smaller than the first compound semiconductor; and a collector made of a third compound semiconductor of a first conductivity type and having a bandgap wider than the second compound semiconductor, the method comprising growing the third compound semiconductor at a higher temperature than a growth temperature of the first compound semiconductor.
According to the embodiment of the invention, while forming the heterojunction of Type I between emitter and base by using for a disordered material such as disordered InGaP for the emitter, and by using an ordered material such as ordered InGaP for the collector, the heterojunction of Type II can be formed between base and collector, and thus, the energy barrier in an interface can be vanished.
As a result, electrons are injected into the collector without being barred by a potential barrier xcex94Ec.
Furthermore, according to the hot electron effect of the electron injected from the emitter, the influence of Auger recombination or alloy scattering can be controlled, and thus, a high current gain can be obtained.
Moreover, the same effect is realized also by using a two-layered structure which consists of materials which are different in the base.
That is, according to the invention, the heterojunction bipolar transistor with which high current gain is obtained at high speed can be realized certainly and easily, and the merit on industry is great.